Which items are worth upgrading in Four Heroes of Light? I'd like to know on two levels, the best all throughout and the ones I should consider upgrading mid-game (just after the world turns dark).
Early in the game, I recommend upgrading Krinjh's Sword and/or King's Shortsword (Knife). They each have both Attack and Magic Attack which will increase with upgrades. For armor, Krinjh's Robe also has Defense and Magic Defense attributes.
Right after the world turns dark, the Lightbringer and Flameblade swords are useful because of their elements (and the Mutsonokami because of its high Attack value). For armor, Rolan's Clothes and Rikoteh's Clothes have both Defense and Magic Defense attributes, as do the Monk Robe and Paladin Armor.
Overall, the best equipment to upgrade overall will depend on two things:
Protip: When upgrading, if you don't like the gems required to upgrade, you can back out of the Upgrade screen and then try again. Different gems will be required the next time you enter the screen.